1. Field of the Invention
The present invention relates generally to surgical ablation. More specifically, the invention relates to a system for aligning bipolar electrodes for radiofrequency ablation, and most specifically to a system for automatically aligning bipolar electrodes on the atrial wall for treating atrial fibrillation by transmural radiofrequency ablation.
2. Description of Related Art
Atrial fibrillation (“AF”) is a heart disease that affects one to two percent of the population of the United States. It is estimated that at any given time, about 2 million people in the United States experience some form of AF, and about 160,000 new cases are diagnosed annually. The prevalence of AF and the health risks associated with it increase with age.
In a patient with AF, the electrical impulses that are normally generated by the sinoatrial node are overwhelmed by disorganized electrical activity in the atrial tissue, leading to an irregular conduction of impulses to the ventricles that generate the heartbeat. The result is an irregular heartbeat, which may be intermittent or continuous. In human populations, AF-induced irregular heartbeat is a significant source of stroke, heart failure, disability, and death.
There are a number of surgical options available for treating AF. One approach pioneered by Dr. James Cox and associates was first performed in 1987, and after several refinements, has evolved into what is now widely known as the Cox-Maze III procedure. In this procedure, the left atrial appendage is excised, and a series of incisions and/or cryolesions are arranged in a maze-like pattern in the atria. The incisions encircle and isolate the pulmonary veins. The resulting scars block the abnormal electrical pathways, improving normal signal transmission and restoring regular heart rhythm. While the success rate is relatively good, the Cox-Maze III and variations thereof are complex open-heart surgeries, requiring cardiopulmonary bypass, median sternotomy, and endocardial incisions that require suturing of the atria. The risk of complications from Cox-Maze III remains significant.
More recently, less invasive techniques have been proposed that use heating or cooling sources to create impulse-blocking lesions on the heart by ablation rather than incision. For example, a procedure known as microwave minimaze, which may be performed epicardially, uses microwave energy to destroy electrical pathways in the atria by heating the tissue at the resonant frequency of the water molecule. In this procedure, small incisions are made on each side of the chest for inserting surgical tools and an endoscope. A flexible microwave antenna is moved along guide catheters into position behind the heart and energized. Aided by the endoscope, the surgeon guides the antenna along the atria to create the pattern of lesions around the pulmonary veins. Clinical research indicates that microwave ablation has a relatively high success rate of about 80%, and allows for the creation of transmural rather than superficial scars. However, the resonance effect of the microwave can be difficult to control, resulting in variable scar formation, and can cause unwanted damage to surrounding tissue.
Other ablation techniques have been developed that use a combination of incisions, cryoablation, and unipolar or bipolar radiofrequency (“RF”) energy to create the pattern of lesions achieved in the original Cox maze procedure. The cryoablation technique per se has seen limited use due to the rigidity of the cryoprobes and to the technical difficulties inherent in the procedure. Unipolar systems have been used successfully in epicardial procedures on a beating heart. However, the transmural lesions created using a unipolar electrode are difficult to control due to the composition of the diseased atrial wall and to the effects of convective cooling from blood flow through the atria. The unipolar RF technique has also been used for ablation in endocardial procedures with somewhat elevated risk factors. Endocardial ablation has been associated with unwanted perforation of surrounding organs, due mainly to the difficulty of achieving consistent burn penetration.
Whether epicardial or endocardial, the unipolar procedure is inherently challenging because it requires that the surgeon move the electrode from point to point and effectively connect the dots to create a desired burn path. If the electrode is moved too slowly, prolonging the burn time at any one point, excessive tissue may be destroyed. If the electrode is moved too quickly along the burn path, or if it is inaccurately placed, gaps may occur in the lesion scar and the abnormal electrical pathways that cause AF may not be completely interrupted. In the latter case, a surgeon may need to repeat the maze procedure one or more times, thereby multiplying the risk factors. In about half of all cases, a surgeon must repeat the ablation procedure one or more times to achieve the desired results.
Bipolar RF ablation is becoming more common. It is effective in creating transmural scars and among all procedures has the best current success rate of about 80% to 90% for treating AF. Many problems, however, can arise from this procedure and lead to further complications. The electrodes used for bipolar ablation are typically clamps, which can be placed on the inside or outside of the atrium to burn a lesion into the clamped area of tissue. Use of the clamp on the inside atrial wall, however, requires opening the atrium to accommodate the clamp. The use of two point electrodes in a bipolar procedure is considered impractical for transmural ablation, as the surgeon would need to effect simultaneous placement of an endocardial and an epicardial probe, and maintain precise control over the speed and placement of the electrodes. If the placement pattern is inaccurate, an excessive amount of atrial tissue may lie within the burn path, and result in unnecessary destruction of tissue.
Some of the more serious complications that can arise from any of the foregoing ablation procedures are those caused by time dependent deep heating through excessive heat transfer. A perforation of the atrial wall due to excessive heating can cause permanent structural damage to the heart, or to the heart and to surrounding tissue. In one scenario, a perforation of the heart can cause a pericardial effusion or cardiac tamponade, which can be fatal without immediate evacuation of the pericardial cavity and corrective surgery. In another scenario, excessive heat transmitted by RF energy or microwaves can permeate the thin wall of the left atrium and fuse it with the esophagus, forming a fistula between the two organs. This creates a pathway into the heart for bacteria from the esophagus, posing a significant risk of infection, endocarditis, systemic sepsis, and mediastinitus outside the heart and in the heart itself. Excessive burning can also injure the endothelium, causing a blood clot that can embolize and lodge in another blood vessel or in the brain and cause a stroke or heart attack.
More recently, to minimize the risk of esophageal injury from excessive heat transfer, complex safety precautions are employed in conjunction with unipolar RF ablation. These include the use of proton pump inhibitors, fluid hydration, esophageal mapping, imaging, temperature monitoring, and energy delivery optimization. To optimize energy delivery, lesions are created by applying higher power over a shorter time period to the ablation site. For example, one technique employs a point electrode mounted in an irrigated catheter tip. The electrode is energized with a continuous RF current to deliver about 50 Watts to the ablation site, and the catheter is dragged across the atrial wall for a duration of about 2-5 seconds. The short duration minimizes the risk of time dependent deep heating. However, the surgeon must pass the electrode along the same lesion path multiple times to achieve a desired result, and must wait about two minutes between each pass. This undesirably prolongs the procedure.
What is needed to improve the efficacy of surgical and interventional vascular treatment for AF is a more reliable and less invasive means for controlling the RF energy during ablation of the atrial tissue.